Dry friction clutches are useful for allowing selective engagement of an input shaft of a gearbox. The clutch assembly typically includes a pressure plate that is axially movable towards and away from a flywheel coupled to an output shaft of an engine, and a friction disc positioned between the pressure plate and the flywheel and coupled to an input shaft of a gearbox. The pressure plate is normally urged toward the flywheel to maintain engagement between friction disc and the flywheel to transmit power from the output shaft of the engine to an input shaft of the gearbox. The friction disc has friction surfaces on opposite sides of the disc which typically engage annular friction surfaces on the flywheel and the pressure plate, respectively, when the pressure plate is urged toward the flywheel. In a vehicle having a manual transmission, a clutch pedal is pressed to cause the pressure plate to be axially moved away from the flywheel to disengage the output shaft of the engine from the input shaft of the gearbox and allow a different gear in the gearbox to be selected for engagement of the gearbox input shaft to a gearbox output shaft.
When the friction disc is brought into engagement with the flywheel and pressure plate there is a short period when the engaging friction surfaces contact each other but are not fully engaged allowing relative movement of the friction disc with respect to the flywheel and pressure plate. During these short periods in which relative movement of contacting friction surfaces occurs, abrasion and generation of particles occurs. These particles can become trapped between the engaged friction surfaces on one side of the clutch disc and the flywheel, and between the engaged friction surfaces on the other side of the clutch and the pressure plate, causing a reduction in friction, which can lead to slippage and further abrasion. Outgassing during this period can also cause reduced friction.
Known solutions have generally involved adjusting clutch operation to increase the force urging the pressure plate toward the flywheel. To the knowledge of the inventors, there are not any known structural features for facilitating removal of particles and gasses that become trapped between the friction surfaces of the flywheel and friction disc or between the friction surfaces of the pressure plate and the friction disc.